As an example of an impact detecting device for detecting an impact applied to an object equipped to a vehicle, a publication of Japanese Patent No. 4962863 (JP 4962862 B2) discloses an impact recording body for a fuel tank.
The impact recording body is made of austenite steel that creates phase transformation upon reception of an impact. The impact recording body covers a fuel tank. When an impact is applied to the fuel tank, the phase transformation of the austenite steel occurs in the impact recording body, and martensite is generated by the phase transformation of the austenite steel. The magnitude of the impact is detected by measuring the quantity of the martensite using a permeameter. Therefore, it is possible to determine whether the fuel tank has received an impact which causes damage to the fuel tank, based on the quantity of the martensite.
In the method where the impact applied to the fuel tank is detected using the impact recording body, it is necessary to measure the quantity of martensite using the permeameter. Therefore, it is difficult to immediately know whether the fuel tank has received the impact. Further, even if the fuel tank has received an impact that causes damage to the fuel tank, a user will not know such damage until a periodic inspection.